Permanent tracers installed in producer wells have by the applicant Resman and others been proven for estimating “what flows where and how much”, i.e. which fluids flow in which parts of the well, and at which flow rates. Traditionally, different tracers have been placed in different influx zones to a production completion installed in a well. These tracers are normally initially immobilized, but they will release as a function of downhole properties like flow velocity, by the affinity to different fluids. Topsides sampling and analysis of the concentration curves over time of the different tracers is used to provide information on which fluids are flowing into which well zones, and may in some cases also indicate at which rates the influx occurs in those influx zones.
In the present context, a tracer carrying system (2) is an injector unit which releases tracer molecules or particles (3), such as a cylinder filled with a tracer carrying fluid and a piston that can drive out the molecules or particles (3) according to some control. By this method tracer clouds are mobile immediately after being injected and able to be transported with the fluids they are injected into. Such a tracer injection system for downhole use is described by many and U.S. Pat. No. 6,840,316 B2 is one such document where tracers are described as being injected into many different positions in well systems and where tracer concentrations are recorded somewhere downstream to enable the estimation of information related to inflow profiles. The injections are always done into parts of the main flow path of the well. What is new in this invention is that tracers are not injected directly into the main flow path of the well, but rather into flow shunts wherein the flow is a function of the main well flow and its pressure gradient. These flow shunts will have flow velocities that are different and normally lower compared to those in the main flow path. However, there should always be a deterministic relationship between the shunt flow and the main flow. Due to these facts, such shunt chambers are regularly referred to as Tracer Delay Chambers (TDC). TDCs are voids inside completions or volumes of gravel and formation where a cloud of tracer molecules or particles will have larger Residence Time Distribution (RTD) than in volumes in the main well flow path. The applicant has during 300 well installations accumulated knowledge from the usage of constantly releasing tracer carrying systems that points towards the fact that transient tracer responses from TDCs created during flow transients will represent the Residence Time Distribution (RTD) in the Tracer Delay Chambers (TDC) and therefore also the rate through it. The larger Residence Time Distribution (RTD) in the Tracer Delay Chambers (TDC) will lead to slower flush-outs of the tracers and thus longer tracer clouds travelling to surface. This is a benefit since smaller tracer clouds will more tend to be distorted by dispersion phenomena in the well hydraulics.
In this context, a base pipe is an established term for a central pipe in a production well, usually of steel, but which may be made in other materials. The Central pipe is an inner pipe into which the production fluid enters in the production zone, and which leads downstream all the way up/out to topside, although there may be some rearrangement of the piping at the wellhead.